"Deer" is the same in singular and plural. "Beer," as a type of drink, is always singular (e.g. "The beer they brew is the best." "There is some beer left in that keg."). However, when we say "beer" and mean "a bottle/can of beer," then this noun takes the normal plural form: 1 beer, 3 beers, 5 beers (I guess that's my limit :).

Deer is an exception for plural in English language, but to understand beer, it is a type of drink just like wine or Pepsi, contained in an object, say bottle or glass. So when using the plural form, we can say two beer because the plural form is actually used for the container, say two pints of beer or three bottles of Coke.